1. Technical Field
This disclosure generally relates to switching regulators for a whole category of electronic equipment installed in computerized personal organizers, handsets, voice recognition devices, voice memory devices, computers, etc.
2. Discussion of the Background
Current-mode switching regulators are widely used in portable electronic equipment, such as mobile phones, digital cameras, and the like. Currently, a switching frequency of the switching regulators is typically lowered while the electronic equipment is in a standby state in order to save energy.
FIG. 12 illustrates an example of circuitry of a known current-mode switching regulator, and FIG. 13 is a timing chart illustrating examples of waves of signals generated in the switching regulator shown in FIG. 12.
Referring to FIG. 12, a switching regulator 100 steps down an input voltage Vin input to an input terminal IN to a predetermined or given voltage and outputs an output voltage Vo from an output terminal OUT, while a detection circuit 101 converts an inductor electrical current i101 to voltage and outputs an electrical current detection voltage Vi. It is to be noted that hereinafter, electrical current is simply referred to as current.
The switching regulator 100 further includes an oscillator circuit 102, a RS (Reset Set) flip-flop circuit 103, a differential amplifier 104, a comparator 105, a reference voltage generation circuit 106, an inverter 107, an inductor L101, switching elements M101 and M102, an output capacitor C101, and a load 110. The switching element M102 is for synchronous rectification.
When the output voltage Vo decreases, a differential voltage Ve output from the differential amplifier 104 increases, which extends a time period required for a current detection voltage Vi to rise above the differential voltage Ve. Consequently, the switching element M101 is kept on for a longer time period, increasing the output voltage Vo. By contrast, when the output voltage Vo increases, the switching element M101 is kept on for a shorter time period, decreasing the output voltage Vo.
Thus, the output voltage Vo is adjusted to a predetermined or given voltage by controlling a time period during which the switching elements M101 and M102 are complementarily kept on and off according to changes in the output voltage Vo.
However, in known synchronous rectification switching regulators of current-mode control type such as the one shown in FIG. 12, when the output current io abruptly changes, the output voltage Vo significantly changes due to a delay in response of a feedback control system.
More specifically, referring to FIG. 13, when the output current io abruptly changes at a time point T0, the output capacitor C101 supplies electrical charge to the load 110 corresponding to the change in the output current io, and thus the output voltage Vo decreases. When the output voltage Vo decreases, a differential voltage Ve increases, and accordingly the switching element M101 is kept on for a longer time period. That is, although the switching element M101 is controlled to increase a peak value of the inductor current i101, the differential amplifier 104 fails to respond promptly due to a phase compensation circuit, etc., provided therein. Thus, the differential voltage Ve fails to change promptly, causing the output voltage Vo to decrease significantly.
When the output voltage Vo changes significantly, the output voltage Vo might decrease to below a specified operating range of the load 110 that is connected to the output terminal OUT, causing failure of the load 110.
To solve this problem, a known switching regulator includes a feed forward circuit that detects changes in the output current when the output voltage changes significantly, and the amount of the detected change in the output current is added to a detection signal of an inductor current. Thus, the inductor current can quickly change in response to the abrupt change in the output current, and accordingly fluctuations in the output voltage can be kept relatively small.
Another known switching regulator detects an abrupt change in the output voltage using a differential circuit that differentiates the output voltage with respect to time, and an output voltage from the differential circuit is added to a differential voltage generated by a differential amplifier so as to compensate for a delay in response of the differential amplifier, reducing fluctuations in the output voltage.
However, in the case of the switching regulator including the feed forward circuit described above, at least a current detector, such as a resistor, connected in series with the load, is required in order to detect changes in the output current.
Further, in either of the above-described switching regulators, when changes in the output voltage are detected while the switching element is off, countermeasures cannot be taken until the switching element is again turned on, causing a decrease in the output voltage.
In particular, when the electronic equipment is activated from the standby state, a load current can quickly increase from several micron amperes to several hundred amperes. However, because the switching frequency of the switching regulator is lowered while the electronic equipment is in the standby state as described above, it can take a relatively long time period from when the output voltage is decreased due to an increase in the output current to when the switching element is again turned on, causing the output voltage to decrease significantly.
In view of the foregoing, it is preferred to stabilize the output voltage immediately when detecting a decrease in the output voltage of a predetermined or given value or greater, which the known switching regulators fail to do.